Many useful processes are based on separations made by selective permeation through relatively thin porous membrane surfaces. The porous surfaces may be in the form of sheets, tubes or hollow fibres and many shapes and configurations are used.
The growing variety of applications for membranes arises from the fact that separation by a well-designed membrane uses only a little more than the theoretical energy minimum required by the entropy decrease due to separation. The efficiency o separation depends upon the perfection and specificity of the membrane to the application and is at its most selective in biological separations. There is, therefore, a developing need for a great variety of membrane topologies and chemistries formed from a great variety of substrates to gain selectivity, productivity and economy.
Recently, thermoplastic (and hence soluble and fusible) polymers have been widely used for the formation of membranes. Such polymers may be formed into many pore size distributions by many processes. However, the thermoplastic nature which makes initial formation of the membranes an easy task can, in some circumstances, become a disadvantage when the membrane is in use. In order to overcome the problems of heat distortion and inadequate solvent and chemical resistance, the thermoplastic polymers require cross-linking into a thermoset form.
Some thermoplastics, such as polyamides, can be cross-linked after forming but the products are still adversely affected by chlorine and hypochlorite which are used for sterilisation of the membranes.
Many of the useful cross-linked resins such as epoxy-resins, phenolic resins and polyurethanes are not available in fine porous barrier form in which the pore sizes range from 0.001 to 1 micron although coarse blown open-celled foams do exist. Similarly, fine ceramic porous barriers are limited in availability.
Although some materials such as stabilised zirconias and silicas have been formed by sintering into porous aggregates, they have not been obtained in any membrane form. Siliceous diatom skeletons occur naturally but they have not been obtainable in large, shaped forms having controlled pore size so as to be usable as a membrane.
Japanese Patent No. 55 162,466 (1981) discloses solid slurry outline coarsely-pored artefacts produced when alumina is pasted onto a necessarily compartatively large-pored polyurethane open foam which is then removed by burning. Such prior art is not directly relevant to this invention and even less relevant is the old Welsbach cerium oxide gas mantle pseudomorph artefact which is a non-reversed phase residue.